Chemical vapor deposition by thermal decomposition of a metal carbonyl has long been a known method of plating substrates. A substrate within a chamber containing a metal carbonyl gas is heated to a temperature above the decomposition temperature of the metal carbonyl gas. The metal carbonyl gas decomposes, coating the substrate with a metal coating, such as a nickel coating from decomposed nickel carbonyl gas. Useful metals which may be deposited from the metallic carbonyl compounds are nickel, iron, chromium, molybdenum, tungsten, cobalt, tellurium, rhenium, and the like.
Methods of heating a substrate in chemical vapor deposition chambers include induction, electrical resistance, and infrared heating. Infrared heating is required when the substrate to be heated is not electrically conductive, such as polyurethane. Infrared heating is directed into the chamber through infrared transparent windows to heat the substrate. Preferably, infrared heating selectively heats the substrate within the chamber, not the metal carbonyl gas or the infrared transparent window. If the metal carbonyl gas is heated above its decomposition temperature, it spontaneously decomposes. Similarly, if the infrared transparent window is heated above the decomposition temperature of the metal carbonyl gas, the gaseous compound decomposes on the infrared transparent window. This situation is referred to as "fogging" of the window. Fogging of the window results in a stoppage of carbonyl plating of substrates because the radiation can not effectively penetrate the fogged windows. To solve this problem, the windows must be removed to be cleaned or replaced.
One source of this problem is that infrared transparent windows constructed of materials such as borosilicate glass, clear fused quartz, polyethylene, terephtalate, polytetrafluoroethylene, polytetrafluoroethylenepropylene and other materials are not perfectly infrared transparent. Therefore, infrared transparent windows are heated by the absorption of infrared radiation, whereby windows may be heated to temperatures at which decomposition of metal carbonyl gas occurs. Metal carbonyl gas decomposes on the overheated windows fogging the windows. Once fogging occurs, the deposition process rapidly accelerates. Fogging accelerates because, the metal product of the carbonyl gas also is not completely infrared transparent which causes increased infrared heating of the windows.
Jenkin, in U.S. Pat. No. 3,213,827 describes an air cooled cooling duct for cooling an infrared transparent chamber wall. However, due to the inefficiencies of air cooling, it is believed that the design of Jenkin is insufficient to effectively prevent fogging.
It is an object of this invention to effectively prevent fogging of infrared windows in carbonyl decomposition chambers.
It is another object of this invention to effectively prevent fogging with a minimal loss of operating efficiency.